A study of the organization of proteins in endoplasmic reticulum (ER) membranes will be carried out on isolated rough and smooth microsomes derived from rat liver. The set of microsomal proteins will be characterized as to its peptide composition. The arrangement of proteins with respect to each other and with respect to the phospholipid bilayer will be described in an assembly map. Proteins exposed on each side of the membranes will be recognized by lectin binding and accessibility to proteases and by selective labeling techniques. Crosslinking reagents will be used to establish near neighbor relationships, between proteins, and to recognize and isolate proteins involved in the binding of ribosomes to the memebranes. Membrane perturbing agents such as detergents with different polarity and hydrophobic to hydrophilic ratios will be used to remove selectively membrane components. After omission of specific components or groups of components in vitro reassembly experiments will be performed to establish their role in dictating or maintaining membrane organization. The content of the microsomal cavities will be analyzed to determine the nature, function and fate of its components. The site of biosynthesis of membrane proteins in free or bound polysomes will be correlated with their location within the membrane.